Reversible Axial Fan: Changing Direction and Choosing the Motor

There is no separate motor type called a “reversible fan motor”: the motor driving a reversible axial fan is a standard three-phase induction motor, and it turns the other way when two of its supply phases are swapped. What makes the fan reversible is the impeller, not the motor: reversible fans use a symmetrical blade profile, so air is moved in both directions at roughly the same flow rate. On the motor side, three things matter: the impeller must stop before the direction changes, the panel or inverter arrangement that performs the reversal, and the shaft power of the fan in its most demanding direction.

Changing the direction of rotation is electrical work. Before touching the terminals, contactors or inverter wiring, the supply must be switched off and locked out, and the work must be done by a qualified electrician; the steps in this article are for planning, not installation instructions.

DRG Motor is a manufacturer, supplier and seller of electric motors; you can find 1000, 1500 and 3000 rpm three-phase motors suitable for reversible fans, listed by frame and speed code, on our Buy Electric Motors Online page. For an overview of fan types, see our Fan Motors page.

What is a reversible axial fan and where is it used?

A reversible axial fan moves air parallel to the shaft axis and fully reverses the airflow when the direction of rotation changes. The word “reversible” does not describe an operating principle or a housing type: the principle is still axial, and the housing is usually a cylindrical casing or a jet fan casing. Reversibility is a property of the impeller and of the operation, chosen according to the purpose.

Typical applications:

• Timber and brick drying kilns: the air is reversed at intervals so both sides of the load dry evenly [4]. We cover kiln temperature and motor placement separately in our drying oven circulation fan article.
• Potato, onion and grain stores: the pile is ventilated from both sides without moving the fan [4].
• Tunnel and car park ventilation: air is pushed one way or the other depending on traffic, wind or the fire scenario [2]. General motor needs on the tunnel side are in our mine and tunnel ventilation motors article.
• Greenhouses and livestock buildings: the same fan supplies or extracts.

Why is running a normal axial fan backwards not the same thing?

A standard axial fan blade has an aerofoil-like profile: a thick, rounded leading edge and a thin trailing edge, with the blade angle set for one direction. When the motor runs in reverse, air does flow the other way, but the blade now meets the air with its thin edge. The result is lower flow, lower pressure, more noise and vibration. In a process where the direction changes regularly, this is not acceptable.

In a reversible impeller both edges have the same shape and the profile is symmetrical. One manufacturer’s catalogue gives the reverse airflow of such an impeller as 95–100% of the forward airflow [1]; another manufacturer states that it uses a truly symmetrical blade section for the same performance in both directions [2]. The price is a somewhat lower fan efficiency than a one-way profile of the same diameter, so the motor for the same duty may come out slightly larger.

There is one more difference: in a direct-drive fan the motor sits inside the casing. In one direction the air passes through the impeller first and then over the motor; in the other it hits the motor first. That is why some catalogues treat “impeller to motor” as the standard direction and the reverse as a separately specified option [1]. When selecting the motor, always ask the fan manufacturer which of the two directions requires more power.

The motor’s role: direct drive and division of responsibility

In most reversible axial fans the impeller is mounted directly on the motor shaft; there is no belt or coupling. The motor sits in the airstream and is cooled by the air passing over it. That air is hot and humid in a kiln, dusty in a store and laden with exhaust gas in a tunnel; protection rating and terminal box position are therefore considered together with the environment.

Fan selection, the fan curve in both directions, blade angle and casing design are the job of the fan manufacturer or the project engineer. The motor supplier’s job is to provide a motor that matches the given shaft power, speed, mounting arrangement and ambient conditions. If there is no fan label, the power is not chosen by guesswork but from data obtained from the fan side.

Mounting and motor connection

Mounting the fan in a duct, on a wall or on a ceiling is a different matter from the motor’s IEC mounting type. In cased reversible fans the motor is usually fixed to support arms inside the casing either foot-mounted (B3) or flange-mounted; the exact type is taken from the fan manufacturer’s drawing. In fans that reverse, two details matter: the impeller hub is driven on the shaft by a key, and held axially by a screw and washer at the shaft end [2]. In this joint, which sees torque in both directions, the screw must be locked and the hub must sit on the shaft without play. The terminal box should stay outside the casing and the cable must be secured in the airstream. To confirm dimensions, use our technical drawings page.

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How do you change the direction of rotation of a fan or extractor motor?

In a three-phase induction motor the direction of rotation is set by the phase sequence of the rotating field. With standard terminal marking, when phases L1, L2, L3 are connected to terminals U1, V1, W1 in that order, the motor turns clockwise viewed from the shaft end [3]. Swapping any two phases reverses the direction. The method depends on how often and how the direction is to change:

Points to watch in a forward-reverse contactor panel

If both contactors pull in at the same time, the result is a phase-to-phase short circuit; that is why both mechanical and electrical interlocking are used. If the panel also has star-delta starting, the phase swap is made at the input of the main contactor, i.e. before the star and delta contactors; mixing up the leads on the terminal side creates a short-circuit risk at the changeover to delta. We explain the terminal layout in detail in our star and delta connection article.

The impeller must stop before reversing

If reverse is commanded while the impeller is still turning, the motor plug-brakes: it draws a current close to or above its starting current, and the shaft, key and impeller hub take a sudden torque shock. Overload trips, burnt contactor contacts and a loosened hub are the typical results of this habit. The solution is a timer relay delay between the two direction commands. The delay is not guessed; it is set by measuring how long the impeller takes to stop completely after the fan is switched off, and adding a margin. On a fan exposed to wind, the impeller can also be turned from outside, so this time is kept longer.

Reversing with an inverter

With an inverter, the direction changes by a parameter or a digital input; the motor wiring is not touched. The inverter first ramps the speed down, passes through zero and accelerates in the opposite direction. During deceleration, the impeller’s kinetic energy drives the motor as a generator and raises the DC bus. If the ramp is too short, the inverter trips on overvoltage; either the ramp is lengthened or a braking resistor is used. See our article on inverter dynamic braking and braking resistors.

Motor selection: required data, power and number of poles

Checklist

• Flow rate and static pressure for both directions (fan curve or label)
• Impeller diameter, number of blades and blade angle
• Fan design speed and which direction needs more power
• Frequency and method of reversal: contactors or inverter
• Impeller run-down time (measured) and wind effect
• Air temperature, humidity, dust; whether the motor sits in the airstream
• Shaft diameter and length, impeller hub, mounting type (fan drawing)
• Whether the fan also has a smoke extraction duty

The last item is important: if the fan also extracts smoke during a fire, the fan and motor set must be certified to EN 12101-3. DRG’s standard motors are not sold with this certification; a DRG motor can only be recommended for normal ventilation duty that does not require certification.

Power calculation: an example produce store

We do not give a single power range for reversible fans; power changes with diameter, flow, pressure, blade angle, speed, air density and system resistance, so the range is determined by application data. Fan shaft power is calculated as P_shaft = Q·Δp / η_fan. Example: in a potato store, 16,200 m³/h (4.5 m³/s) of air is to be passed under the pile, the highest static pressure in either direction is 300 Pa, and a fan efficiency of 0.60 is assumed. The shaft power is 4.5 × 300 / 0.60 = 2,250 W, i.e. about 2.25 kW. Direct drive has no belt loss; adding a 10–15% margin gives 2.5–2.6 kW, so the next standard step, 3 kW, is selected. The flow loss in reverse and the lower fan efficiency of the symmetrical profile are already part of this assumption; if the fan manufacturer gives a different value, its curve takes precedence. To calculate with your own values, use the Pump & Fan Motor Power calculator.

Speed and poles: the side specific to reversible fans

The blade tip speed of an axial impeller is v = π·D·n/60. A 1000 mm impeller reaches a tip speed of about 76 m/s on a 4-pole motor (1450 rpm) and about 51 m/s on a 6-pole motor (968 rpm); on a 2-pole motor it reaches 153 m/s, above what most impellers of this diameter allow. The fan curve decides the number of poles, but a reversible fan has one more criterion: the rotational energy that must be removed at each reversal rises with the square of speed. Running the same impeller at 968 instead of 1450 rpm cuts this energy by about 2.2 times; run-down is shorter, and the inverter ramp and braking resistor become smaller. For fans that reverse often, a lower-speed solution is therefore worth considering if the fan curve allows it.

For the 3 kW requirement in the example, two options stand out: the 4-pole DRG-100L2-4 draws 6.02 A at 400 V and 1450 rpm; the 6-pole DRG-132S-6 draws 6.84 A at 968 rpm and is larger, in frame 132S. The fan design speed and the space inside the casing decide which one fits. The cooling fan of standard enclosed motors is generally designed to work in both directions; still, state in your order that the motor will be used in both directions. The DRG range covers 0.25–11 kW in aluminium frames and 0.55–355 kW in cast iron frames; three-phase, 230/400 V or 400/690 V, supplied in B3, B5, B14 and B35 mounting with an order note. Not every reversible fan power on the market matches this range one to one; confirm with the fan drawing and curve.

DRG motors for reversible axial fans

Models to consider for reversible axial fans around the 3 kW of the example:

• DRG-100L1-4 – 2.2 kW – 1500 rpm – 100L
• DRG-100L2-4 – 3 kW – 1500 rpm – 100L
• DRG-132S-6 – 3 kW – 1000 rpm – 132S
• DRG-112M-4 – 4 kW – 1500 rpm – 112M

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Reversible fan motor: FAQ

How do you change the direction of rotation of a fan motor?

On a three-phase motor, two of the supply phases are swapped; on an inverter-fed system the direction command is given by a parameter or a digital input. The work must be done by a qualified electrician after the supply is switched off and locked out.

Does every axial fan become reversible when run backwards?

No. Air flows the other way, but with a one-way profile flow and pressure drop noticeably and noise rises. Regular reversal requires a reversible impeller with symmetrical blades.

Does a reversible fan need a special motor?

Usually not; a standard three-phase induction motor runs in both directions. What matters is that power, speed, mounting and ambient conditions suit the fan, and that the impeller stops before reversing.

Why does the overload trip when reversing?

When reverse is commanded while the impeller is still turning, the motor plug-brakes and draws a current close to its starting current. A measured delay between the two direction commands, or a longer inverter ramp, solves it.

Send us your fan label, the old motor’s nameplate or the fan drawing and we will determine the right motor together: +90 (542) 666 11 11, [email protected], contact page. For current prices, browse the 120 models in our online store.

Sources

1. Kruger – TRC Series, Direct Drive Reversible Vane Axial Fan, Product Catalogue – 95–100% airflow in reverse; standard flow direction
2. Woods Air Movement – JMTS Reversible Axial Flow Fan, Specification Sheet – symmetrical blade section, impeller fixing on the shaft
3. IEC 60034-8:2026 – Rotating electrical machines, Part 8: Terminal markings and direction of rotation – phase sequence and direction of rotation
4. Multi-Wing – R-Series reversible axial fans – drying kiln and crop store applications